The present invention relates to piezoelectric crystalline films. More particularly, it relates to piezoelectric crystalline films comprising zinc oxide having a hexagonal crystal structure.
There are many methods for making piezoelectric zinc oxide films such as, for example, vacuum deposition methods, epitaxial growth methods, and sputtering methods. Among these methods, the sputtering methods, particularly, a radio-frequency sputtering method has been used very often lately because of a advantage that a growing rate of oriented crystalline films is high, thus making it possible to mass-produce piezoelectric crystalline films industrially.
When making a piezoelectric crystalline film of zinc oxide on a substrate surface by the radio-frequency sputtering method, ceramics of highly pure zinc oxide has been conventionally used as a source of the film material. However, even when radio-frequency sputtering is effected with such a film material source, it results in the formation of a crystalline film with a rough surface, thus making it impossible to produce a good piezoelectric crystalline film. In addition, it is difficult with such a film material source to make a c-axis perpendicularly with respect to the substrate surface. If a piezoelectric crystalline film of zinc oxide has a rough surface, various disadvantages occur. For example, when manufacturing an acoustic surface wave filter with such a zinc oxide film, it is difficult to form interdigital transducers on the film surface, and the produced acoustic surface wave filter tends to have the disconnection of interdigital transducers, and possess a large propagation loss of acoustic surface waves. Also, if the c-axis of the zinc oxide film is inclined with respect to the axis perpendicular to the substrate surface, the value of the electromechanical coupling factor becomes small, thus making it difficult to produce a piezoelectric crystalline film transducer with good conversion efficiency.
It has now been found that the use of ceramics of zinc oxide containing vanadium makes it possible to produce a piezoelectric crystalline film with a c-axis perpendicular to the substrate surface and a smooth surface.
It is an object of the present invention to provide an improved zinc oxide piezoelectric crystalline film which overcomes the aforesaid disadvantages.
According to the present invention, there is provided a piezoelectric crystalline film of zinc oxide with a c-axis perpendicular to the substrate surface, characterized in that said crystalline film contains 0.01 to 20.0 atomic percent of vanadium.
The piezoelectric crystalline film of the present invention may be made by any conventional methods such as for example, the radio-frequency sputtering methods, the co-sputtering methods and the ion implanting methods.